The present invention relates to a pressure sensor for measuring a pressure by detecting a change in electrostatic capacitance.
In general, a chip-like capacitance type pressure sensor for detecting a pressure by detecting an electrostatic capacitance has a substrate, a diaphragm which forms a space together with the substrate, a stationary electrode disposed on the substrate, and a movable electrode fixed to the diaphragm to oppose the stationary electrode. In the pressure sensor chip having this arrangement, when the diaphragm is displaced upon reception of a pressure, the distance between the movable and stationary electrodes changes to change the electrostatic capacitance between them. The pressure applied to the diaphragm can be measured on the basis of this change in electrostatic capacitance.
As a housing comprised of the substrate and diaphragm of such a pressure sensor chip, one made of sapphire (artificial corundum) is proposed. When sapphire is used to form the housing, even if the measurement target is a corrosive body or fluid, the housing can directly receive the body or fluid with its diaphragm to measure the pressure.
FIGS. 4A and 4B show the arrangement of a conventional pressure sensor chip.
Referring to FIG. 4A, the conventional pressure sensor chip is constituted by a base 401 and a diaphragm 402. A circular recessed capacitance chamber 401a is formed at the center of the major surface of the base 401. The diaphragm 402 is bonded to the major surface of a rim portion 401b surrounding the capacitance chamber 401a to cover it, thereby forming a closed space in the capacitance chamber 401a. The base 401 and diaphragm 402 are made from sapphire.
A circular stationary electrode 403 is fixed to the bottom surface of the capacitance chamber 401a, and a small disk-like movable electrode 404 is fixed to the lower surface (the capacitance chamber 401a side) of the diaphragm 402 to oppose the stationary electrode 403. The movable electrode 404 is arranged at substantially the center of the diaphragm 402. Furthermore, a ring-like reference electrode 405 is fixed to the lower surface of the diaphragm 402 to oppose the stationary electrode 403. The reference electrode 405 has a ring diameter smaller than the diameter of the stationary electrode 403, and is arranged to surround the movable electrode 404. At the periphery of the capacitance chamber 401a, a lead portion 404a extending from the movable electrode 404 is connected to a pin 406 extending through the base 401, as shown in FIG. 4B.
In the pressure sensor chip having the above arrangement, the opposing stationary and movable electrodes 403 and 404 constitute a capacitor. Hence, upon reception of an external pressure, when the center of the diaphragm 402 is displaced toward the base 401, the distance between the stationary and movable electrodes 403 and 404 changes to change the capacitance between them. This change in capacitance is electrically detected through the lead portion 404a, the pin 406, and the like, so that the pressure acting on the diaphragm 402 can be detected.
The reference electrode 405 formed on the diaphragm 402, and the stationary electrode 403, that are adjacent to each other, also form a capacitance between them. Since the reference electrode 405 is arranged close to the rim portion 401b, the warp amount of the reference electrode 405 accompanying warp of the diaphragm 402 is smaller than that of the movable electrode 404 arranged at the center of the diaphragm 402.
The dielectric constant of air in the capacitance chamber 401a changes in accordance with the humidity, to accordingly change the capacitances of the respective electrodes. If the change in capacitance between the stationary and movable electrodes 403 and 404 is observed with reference to a change in capacitance between the stationary and reference electrodes 403 and 405, the warp amount of the diaphragm 402 can be detected without fluctuation even if the dielectric constant of air in the capacitance chamber 401a changes.
Concerning sapphire used in the substrate and diaphragm constituting the pressure sensor, a sapphire substrate having an R plane as a major surface is used in terms of cost and availability. As shown in FIG. 5, the R plane of a sapphire crystal is a plane that forms an angle of 57.6.degree. with the C plane.
When sapphire crystals are grown by the EFG (Edge-defined Film-fed Growth method) such that the R plane is set horizontal, a sapphire crystal plate having a large area to a certain degree can be obtained comparatively easily. In contrast to this, when crystal growth is performed while pulling sapphire upward in the direction of C axis, an ingot having a large diameter cannot be obtained. For this reason, it is very difficult at the present stage to obtain a crystal plate having a large C plane.
In an inexpensive sapphire substrate having an R plane as the flat surface, its physical properties such as Young's modulus and thermal expansion coefficient are anisotropic. When two sapphire wafers each having an R plane as the major surface are bonded to each other, the bonded wafer body warps unless the axes of the crystals of the respective wafer surfaces coincide with each other. This warp occurs depending on the axis in the R plane described above. This is supposed to be because a change in physical properties of the R-plane wafer caused by the temperature is large in some axis while it is small in another axis.
FIG. 6 shows how the warp occurs in accordance with a temperature change (temperature rise) of the diaphragm in the pressure sensor described above. In FIG. 6, the axis of abscissa represents the distance of a point where the warp amount is measured from the center. The center of the diaphragm is defined as 0, and the rightward direction from 0 indicates a positive value while the leftward direction from 0 indicates a negative value. Note that the warp amount of the diaphragm plotted along the axis of ordinate is a normalized value.
The sapphire diaphragm and base are bonded to each other more firmly when they are heated with their specular polished surfaces being in tight contact with each other. The diaphragm and base are bonded to each other such that their C-axis projection directions of their crystals are displaced from each other by about 10.degree. .
Referring to FIG. 6, block dots represent the warp amount on a line along the C-axis projection direction of the diaphragm, and white dots represent the warp amount on a line forming an angle of 45.degree. with the C-axis projection direction of one diaphragm. White triangles represent the warp amount on a line forming an angle of 90.degree. with the C-axis projection direction of the diaphragm, and solid squares represent the warp amount on a line forming an angle of -45.degree. with the C-axis projection direction of the diaphragm. All of these lines pass through the center of the diaphragm.
In this manner, when a diaphragm comprised of a sapphire R-plane substrate is adhered to a base comprised of a sapphire R-plane substrate such that their axes are displaced from each other (e.g., by about 10.degree.), warping caused by a temperature change occurs. This warp is the maximum on a line forming an angle of 45.degree. or -45.degree. with the C-axis projection direction of the diaphragm, and on an intermediate region between the diaphragm center and the end of the diaphragm.
In the pressure sensor formed by bonding the base and diaphragm, when the C-axis projection direction of the sapphire crystal of the base is different from that of the diaphragm, the diaphragm warps even if no external pressure is applied to it. More specifically, in the conventional sapphire pressure sensor, when the temperature changes, even if no external pressure is applied, a detection signal is detected as if a pressure is actually applied.
When a displacement is present, if any, between the C-axis projection direction of the sapphire crystal of the base and that of the diaphragm, the diaphragm warps upon a temperature change. Meanwhile, the displacement between the base and diaphragm cannot be completely eliminated easily.